Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, but single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they often produce polymers with improved physical properties.
Single-site catalysts commonly incorporate cyclopentadienyl or “Cp-like” ligands such as indenyl or fluorenyl. A recent trend is to use ligands that can chelate to the transition metal with two or more neutral or anionic electron donors, especially heteroatoms. Examples are bis(allyl) dianions (U.S. Pat. No. 6,544,918), N-oxides (U.S. Pat. No. 6,498,121), neutral multidentate azacyclics (U.S. Pat. No. 6,384,229), or quinolinoxys (U.S. Pat. No. 5,554,775).
“Calixarenes” are a well-known class of cyclic oligomers that are usually made by condensing formaldehyde with p-alkylphenols under alkaline conditions. V. Bohmer summarized the chemistry of calixarenes in an excellent review article (Angew. Chem., Int. Ed. Engl. 34 (1995) 713).
Early transition metal complexes in which the four oxygen atoms of calix[4]arenes or O-methylated calix[4]arenes chelate to the metal are now known (see, e.g., J. Am. Chem. Soc. 119 (1997) 9198). These complexes have apparently not been used to polymerize olefins.
Because of their unique topology, complexes in which a calixarene ligand coordinates to a transition metal are potentally valuable for olefin polymerization. Too often, olefin polymerization catalysts based on chelating ligands have poor activity. This is consistent with an energetically favorable trans-coordination of the olefin and growing polymer chain in an octahedral or pseudo-octahedral configuration of active sites. Ideally, the growing polymer chain and complexed olefin would be forced into closer proximity. The calixarene framework creates such an opportunity because the aromatic rings prevent trans-coordination. While known oxy-coordinated calixarene complexes (as described in J. Am. Chem. Soc. 119 (1997) 9198) have the required topology, electron donation from four oxygens to the metal should preclude significant activity.
Biali et al. (J. Org. Chem. 56 (1991) 532 reported a two-step method for making hydroxyl-depleted calix[4]arenes. p-t-Butylcalix[4]-arene is first reacted with diethyl phosphite and triethylamine to give a diphosphate ester. Reductive cleavage of the diester with metallic potassium in liquid ammonia at −78° C., followed by neutralization and chromatographic purification gives a hydroxyl-depleted calixarene (with two hydroxyl groups remaining). Hydroxyl-depleted calixarenes have apparently not been incorporated into transition metal complexes, and they have not been used as components of olefin polymerization catalysts.
The polyolefins industry continues to need new polymerization catalysts. In particular, the industry needs catalysts having activities that rival the activities of single-site catalysts based on cyclopentadienyl, indenyl, and fluorenyl ligands. A valuable catalyst would incorporate ligands that promote a favorable active site configuration in which the complexed olefin and growing polymer chain are forced into close proximity. Ideally, the catalysts could be made economically using well-established synthetic routes.